This invention regards a method of reducing that proportion of volatile organic compounds (VOC) which is separated from oil, in particular crude oil, during the filling of tanks, such as during the loading of a ship from a terminal, a production platform or a floating loading device. The invention also comprises an arrangement for carrying out the method. The arrangement may also be used for reducing evaporation of gas during the filling of single components such as propane, butane, ethane and liquefied natural gas.
Crude oil is made up of different components stabilised at a specified pressure and a specified temperature. If these conditions are altered, either through a reduction in pressure or an increase in temperature, a proportion of the volatile components will separate out and gasify. These components consist of volatile organic compounds such as e.g. methane, propane, butane and ethane, and are termed VOC. Systems currently exist for removal of these gases. As opposed to the arrangement of the present application, today's systems are based on treating the already separated gases by providing a process arrangement for treatment of the exhaust gas subsequent to evaporation. The plants are complex and will require a lot of energy, as pressure and temperature are employed to bring the gases back to a liquid state.
It is common knowledge that a relatively large quantity of volatile organic compounds evaporate during pumping of oil into large tanks. Under normal circumstances, a pressure of the order of 1.05 to 1.07 bar is maintained both in storage and transport tanks. When loading e.g. a tanker, it is customary for the oil to be pumped from a storage tank through a feed pipe to a position above the cargo tank, from where the oil is sent into the tank through a drop line (downcomer) to the bottom of the tank. A drop line of this type may have a length of the order of several tens of metres.
When the oil flows into the upper end portion of the drop line, gravity will accelerate the liquid flowing down through the drop line, whereby a lower static pressure is created in the feed pipe and the upper portion of the drop line. In these pipes, where the static pressure is lower than the vapour pressure, the evaporation of volatile organic compounds is significant, and these compounds will only to a small extent condense back to a liquid state upon resumption of normal tank pressure.